AQUEDUCT (Lat. aqua' ductus, a conduit of water). Broadly speaking, this word means any conduit for conveying water, but usage, both ancient and modern, has practically limited the word to masonry conduits with little or no more slope than is necessary to cause the water to flow through them by gravity. Such limita tions generally exclude mere channels or ditches (canals) in the natural earth, on the one hand, and closed conduits (pipes) under pres sure, on the other. Modern aqueducts are oe casionally, but rarely, under low pressures, and frequently sections of iron or steel pipes under heavy pressure are used to convey the water of hn aqueduct beneath a deep valley. Inverted siphons, as these depressed sections are called, are the modern substitute for the aqueduct bridges of earlier days, or for the circuitous routes necessary to avoid the construction of such bridges. Siphons were not unknown to the Romans, who lacked, however, knowledge of cast-iron pipe, or any other pipe of large size, capable of conveying water under heavy pressure. The general abandonment of the masonry aqueduct for conduits or pipe lines of cast or wrought iron, steel and wood, has re sulted from a variety of causes, such as shorter routes, due to the possibility of taking the most direct path with little regard to hills, valleys, and streams; smaller conduits, due to the in creased velocity that accompanies higher pres sures.; and a consequent diminution in the cost of rights of way. labor, and material. Marked charaeteristies of the modern aqueducts have been great boldness and freedom in the use of the tunnel, and also in the employment of long span arches for aqueduct bridges, or the sub stitution of iron or steel (at present the latter) for masonry bridges. In a few recent instances, where water free. or nearly free from pressure, was to be conveyed, vitrified clay pipes have been employed.
Bearing in mind the foregoing, a brief review of some of the most notable masonry aqueducts of ancient and modern times will be given. Ancient Oriental peoples. such as the Pershing and Phcenieians, used a system of subterranean channels of masonry with vertical shafts at in tervals, such as Pol•bius described (x. 23. 3) for Hecatompylos, the capita] of the Arsacidm. The Pelasgic and Myeenfean cities, such as Mycelia. and Argos, were thus supplied. Herodo tus describes, as one of the most remarkable works of Greek lands, the aqueduct of Samos, built by the engineer Eupalinos with a gallery eight feet square. He also saw at Tyre three aqueducts with arches and viaducts which were imitated at Carthage before the Roman conquest. The early Latin tribes in Italy continued the Pelasgie tradition, as is shown in the famous emissary of the Alban Lake. The water supply
of Athens and its plain can still be studied in a variety of conduits and aqueducts earlier than Hadrian's more striking constructions. The custom of subterranean aqueducts was at first also followed by the R01111111S, whose Appian aqueduct had less than three hundred feet sup ported on arcades above ground. Gradually, with the increase of monumental splendor, com bined with the desire to carry the water to the higher level of the hills of Rome,'a larger per centage of the aqueduct was areaded, and the water brought from a greater distance.
The principle of the inverted siphon was used in such aqueducts as those of Patara, Pergamum, and Aspendos, in Asia Minor, at Constantinople, at Tebessa in Africa, and at Lyons, where it can be studied in great detail; but Vitruvius (vii. 6), in describing this method, warns against it in the case of large volumes of water, whose pressure would not be withstood by the lead or terra-cotta pipes then in use. lu a few eases expensive bronze pipe is used to resist pressure. The careful grading of the aqueduct to prevent a too rapid flow was assisted by curves in the line of construction. This ex plains apparent peculiarities in direction. Tun nels were often cut, sometimes over three miles long. The fail recommended oy Vitruvius is six inches in every one hundred feet, but it was usually greater. At the head of the aqueduct a large reservoir or piscina was established; minor basins were constructed at intervals along the line for filtering and clarifying the water by passing it through gravel. The chan nel tor the water, or species, between two and four feet wide, and four and one-half and six and one-half feet high, was originally of stone, lined with hydraulic cement ; afterwards of con crete faced with brick. At frequent intervals were blowholes through the top or sides, to afford ventilation and access to the interior, and their place was taken in the subterranean sec tions by inspection wells, or putei. The chan nels were large enough to admit the workmen along their entire length for inspection and re pair. Leakages were frequent. and the heavy lime incrustations, if not periodically removed. gradually reduced the size of the channels and the amount of the supply. In many cases sev eral water supplies were carried on the same arches, being joined at a certain distance from their source, and each water being usually car ried in its separate channel. This is the case with the Marcia. which carries also the Tepula and the Julia.